More Evidence That Exercise Reduces Cancer Risk

Three recently published studies find that people
who exercise lower their cancer risk. But all three point out that the exercise
cant be a walk in the park. It needs to be vigorous.

A study of women in China found that those with a
high level of vigorous activity had a lowered rate of ovarian cancer. Reporting
in the International Journal of Cancer (Vol. 105, No. 6: 838-843) Min
Zhang and colleagues compared the physical activity of women with ovarian
cancer to that of women without this disease. They looked at how much effort
the women expended at work or exercise and how much they sweated with these
activities.

Women with a high level of physical activity cut
their ovarian cancer risk in half. This meant they engaged in one of the
following activities:

"vigorous work" like moving heavy
furniture or digging for at least 20 hours a week;

moderate activity such as brisk walking
or bicycling for 30 hours a week;

strenuous sports at least two hours a
week;

working or exercising hard enough to
sweat at least three times a week.

All this seems like a lot of activity and it may
not carry over directly into guidance for women in the US. Chinese women may
normally have a high level of physical activity so they need to work even
harder to lower their risk of ovarian cancer. Also, China's ovarian cancer
rates overall are lower than US rates -- another factor that would complicate a
direct comparison.

Women Cut Breast Cancer Risk with
Exercise

But even U.S. women need to exercise vigorously to
lower their risk of another cancer, breast cancer. A study led by Alpa Patel,
of the American Cancer Society, found that women who were very active lowered
their risk of breast cancer by nearly 30%.

In their report, published in the journal
Cancer Causes and Control (Vol. 14, No. 6: 519-529), Patel and
colleagues examined the relationship of exercise to breast cancer in
postmenopausal women. Although any physical activity appeared to have some
benefit, the 30% reduction in breast cancer rate occurred in women whose
exercise was the equivalent of swimming, running or jogging at least six hours
a week.

Women could achieve the same effect by walking or
dancing; they would just need to do it for twice as long.

Colorectal Cancer Rate Drops with Physical
Activity

In a third study, this time of colorectal cancer,
researchers from the University of Utah and Kaiser Permanente in Oakland,
California, found that both men and women lowered their risk of this disease
with physical activity. In the report, published in the American Journal of
Epidemiology (Vol. 158, No. 3: 214-224) the researchers, led by Dr. M. L.
Slattery, pointed out that vigorous activity provided the greatest benefit.

Men and women who exercised the equivalent of
jogging five or more hours a week lowered their risk of colorectal cancer by
40% to 50%. Some benefit came from less vigorous activity, particularly for
rectal cancer.

Scientists Not Certain How Exercise
Works

The researchers from these three studies all point
out that studying the effect of exercise on cancer risk is an uncertain
science. There may be other characteristics about people that lead them to
exercise and also reduce their risk of cancer. One example might be that their
concern for their health has led them to eat more fruits and vegetables, which
can also reduce cancer risk. Another is that cancer rates are often higher in
obese people and the more active people are generally thinner.

But the researchers didnt think that weight
alone accounted for the differences. There seems to be some biologic connection
between exercise and lowering the risk of some cancers. Although there are many
explanations, none have been proven and scientists are still looking for that
link. People who want to lower their risk for these three cancers benefit from
knowing that vigorous exercise works.

The
American Cancer Society
recommends that adults engage in at least moderate activity for 30 minutes or
more on five or more days of the week; 45 minutes or more of moderate to
vigorous activity on five or more days per week may further enhance reductions
in the risk of breast and colon cancer.